Water absorbent cores containing hydrophilic fiber, such as pulp, and a water absorbing resin are widely used conventionally so that sanitary/hygienic materials, such as disposable diapers, sanitary napkins, and incontinent pads, can absorb body fluids. The water absorbent core is used in sanitary/hygienic materials, such as disposable diapers, sanitary napkins, and incontinent pads, to absorb body fluids.
There are recent demands for these sanitary/hygienic materials to be reduced in thickness for better usability. To this end, water absorbent cores are manufactured with a decreasing ratio of hydrophilic fiber, which has a relatively low bulk density, and an increasing ratio of water absorbing resin, which exhibits excellent water absorption and a relatively high bulk density. The relative quantity of water absorbing resin particles used in the water absorbent core is hence increased, which in turn reduces the thickness of the sanitary/hygienic materials without compromising water absorbency and other physical properties.
The ratio of the hydrophilic fiber may be decreased, but not further below a minimum quantity required. For further reduction in thickness of the sanitary/hygienic materials, the physical properties of the water absorbing resin need to be improved. Examples of such physical properties of the water absorbing resin include centrifuge retention capacity, saline flow conductivity, absorbency against pressure, fixed height absorbency, mass-average particle diameter, and extractable polymer content. The water absorbing resin needs to have these physical properties together in actual use.
These physical properties can be improved by any one of the following four methods: (1) by improving the internal structure of the water absorbing resin, (2) by improving a surface crosslink process, (3) with a liquid permeability improver or other additive, and (4) through the regulation of particle shape and particle size distribution.
Taking the first approach among them, the present invention is intended to improve the internal structure of the water absorbing resin. We have chosen this approach because the improvement of the internal structure is effective not only single handedly, but it also works synergistically with the improvement of the surface crosslink process and the use of additives.
Some technologies are documented that are intended to improve the internal structure. For example, patent document 1 discloses a water absorbing resin that has a particular particle size distribution, particular CRCs, particular AAPs, and a particular chemical crosslink index (or chemical crosslink index under load). The document discloses also a manufacturing method in which a particular polymerization method is used to obtain a water absorbing resin. The resin has a high degree of crosslink, a high retention capacity, and a swelling pressure of gel layer of 35.0 kdyne/cm2 or higher. The resin is processed to exhibit a particular particle size distribution (Particles ranging from 106 μm, inclusive, to 850 μm, exclusive, account for 95 wt % or more of the entire resin content. The particle size distribution has a logarithmic standard deviation σζ of 0.25 to 0.45). After that, the resin is subjected to surface crosslinking, and mixed with a liquid permeability improver. The technology improves gel strength by relatively increasing chemical crosslinking points.
Patent document 2 discloses a method in which alkali metal silicate is added before water-containing gel is dried.
Patent document 3 discloses a method in which two kinds of 2 crosslinking agents are used together.
Patent document 4 discloses a superabsorbent crosslinked polymer material for aqueous liquids which contains a partially neutralized, monoethylenic, unsaturated, acid group-containing monomer, any other monomer copolymerizable with said monomer, and any polymer suited for use as a graft base.
Techniques that are similar to the present invention, but have a different objective are those involving mixing a polymerized water-containing gel with an additive, such as a persulfate. The techniques are intended to lower residual monomers in water absorbing resins and therefore based on a different technical concept from the present invention. The techniques indeed achieve reduction of the residual monomers, but fall short of improving the internal structure of the water absorbing resins due to the quantities of the additives being different and for other reasons. An example of such techniques is the method disclosed in patent document 5. According to the method, a water-containing gel is mixed with fine particles of a water absorbing resin as well as with a polymerization initiator or a reduction agent. Another example is disclosed in patent document 6. According to the method, a water-containing gel is upon comminution mixed with fine particles of a water absorbing resin as well as with a polymerization initiator, such as a persulfate. A further example is disclosed in patent document 7. According to the method, a water-containing gel is mixed with a persulfate. Another example is disclosed in patent document 8 whereby fine particles are upon agglomeration are mixed with a persulfate.
[Patent Document 1] International Application Published under PCT WO2005/27986
[Patent Document 2] European Patent 1137678B1
[Patent Document 3] Specification of U.S. Published Patent Application 2004/0014901
[Patent Document 4] International Application Published under PCT WO97/019116
[Patent Document 5] Japanese Unexamined Patent Publication 05-43610/1993 (Tokukaihei 05-43610)
[Patent Document 6] Japanese Unexamined Patent Publication (Tokukai) 2001-79829
[Patent Document 7] European Patent 1358224B1
[Patent Document 8] European Patent 1690887A